1. Field of the Invention
The present invention relates to an audio broadcast receiving apparatus and to a method for receiving an analog audio broadcast radio wave in a specified frequency band (channel) while, at the same time, receiving and demodulating a digital audio broadcast radio wave such as the terrestrial digital broadcast signal transmitted in parallel with the analog audio broadcast in the particular frequency band thereby to retrieve intended audio information (audio data).
In North America, the IBOC (in-band on-channel) scheme is currently employed, in which a radio wave containing an analog modulated wave in a specified frequency band is transmitted for an analog audio broadcast such as an AM (amplitude modulation) radio broadcast or an FM (frequency modulation) radio broadcast, while a radio wave containing a digital modulated wave for the digital audio broadcast can be transmitted by multiplexing using the scheme called the OFDM (orthogonal frequency division multiplexing) at the same time and in the same frequency band.
In particular, this invention relates to a technique for suppressing the mixing between a broadcast station in the channel desired by the user and a broadcast station in an adjacent channel while receiving the analog audio broadcast and the digital audio broadcast transmitted at the same time using, for example, the IBOC scheme.
2. Description of the Related Art
To facilitate understanding of the problems of the conventional audio broadcast receiving apparatus of the IBOC scheme, the configuration and operation of the conventional audio broadcast receiving apparatus of the IBOC scheme are explained below with reference to FIGS. 1 to 6 that will be described in “BRIEF DESCRIPTION OF THE DRAWINGS” later.
FIG. 1 illustrates a schematic diagram for explaining an outline of the IBOC scheme used for the terrestrial digital broadcast in North America. The explanation refers to the IBOC scheme currently employed in North America, in which the radio wave in a specified frequency band (spectrum mask) of the existing FM radio broadcast is transmitted by multiplexing with the radio wave of the terrestrial digital broadcast in the same frequency band. In this IBOC scheme, generally, the analog modulated wave of the existing FM radio broadcast is transmitted in the frequency band of one of a plurality of channels, while the digital modulated wave of the digital audio broadcast such as the terrestrial digital broadcast is transmitted at the same time in the same frequency band.
Specifically, as shown in FIG. 1, the analog modulated wave of the FM radio broadcast is transmitted in the central band MB in a given frequency band, while the digital modulated wave of the digital audio broadcast is transmitted in the upper and lower sidebands SB of the particular frequency band. The transmission power Ps and the frequency band of these analog and digital modulated waves are limited by the spectrum mask SM standardized by the Federal Communications Commission (FCC).
More specifically, the frequency bandwidth (frequency f along abscissa in FIG. 1) of a given channel is set at about 440 kHz (±220 kHz from the central channel) based on the spectrum mask SM. Further, the frequency bandwidth of the analog modulated wave of the FM radio broadcast is set at about 220 kHz (about ±110 kHz from the central frequency) in the central band MB based on the spectrum mask SM. The frequency bandwidth of the digital modulated wave of the digital audio broadcast, on the other hand, is set at about 90 kHz (about 130 to 220 kHz lower than the central channel for the lower sideband, and about 130 to 220 kHz higher than the central channel for the upper sideband) in each of the lower and upper side bands SB based on the spectrum mask SM.
Further, in the IBOC scheme shown in FIG. 1, the digital modulated wave of the digital audio broadcast is transmitted with a power about 22 dB (−2.5 dB/kHz) smaller than the analog modulated wave of the FM broadcast based on the spectrum mask SM.
The frequency interval between a broadcast station of a given channel and another broadcast station of an adjacent channel is set to 200 kHz. In an actual broadcast receiving apparatus, the effects of adjacent channels are reduced as far as possible by selectively passing the analog modulated wave and the digital modulated wave existing in a frequency band of not more than about 400 kHz (about ±199 kHz from the central channel) using a bandpass filter (FIGS. 2 and 4).
FIG. 2 illustrates a block diagram showing a configuration of the conventional audio broadcast receiving apparatus of the IBOC scheme, FIG. 3 a diagram showing the receiving characteristics of the conventional audio broadcast receiving apparatus of the IBOC scheme, and FIG. 4 a diagram showing the frequency characteristic of the bandpass filter shown in FIG. 2.
In FIG. 2, however, the configuration of the conventional audio broadcast receiving apparatus of IBOC scheme is shown in simple fashion. The analog audio broadcast received by this audio broadcast receiving apparatus of IBOC scheme is considered to include the FM radio broadcast (carrier frequency of 88 to 108 MHz) and the AM radio broadcast (carrier frequency of 530 to 1700 kHz). In the case under consideration, however, the FM radio broadcast receiving apparatus is explained.
The audio broadcast receiving apparatus shown in FIG. 2 includes a tuner 100 for receiving the analog modulated wave of the FM radio broadcast and the digital modulated wave of the digital audio broadcast transmitted using the IBOC scheme, and a demodulator 550 for retrieving the intended audio information by demodulating the receiving signal Sr received through the tuner 100.
More specifically, the tuner 100 of the audio broadcast receiving apparatus shown in FIG. 2 includes a channel select processing unit for selectively retrieving the analog modulated wave and the digital modulated wave in the frequency band of a specific channel from a plurality of channels of the FM radio broadcast received through an antenna AT, and a radio-frequency signal/intermediate frequency signal converter for converting the analog modulated wave and the digital modulated wave (radio-frequency signal) in the frequency band of the particular channel into an intermediate frequency signal. The value of the intermediate frequency of the intermediate frequency signal is set to, for example, 10.7 MHz.
The intermediate frequency signal (the receiving signal Sr) retrieved from the radio-frequency signal/intermediate frequency signal converter in the tuner 100 is passed through a bandpass filter for the intermediate frequency signal (hereinafter referred to as the IF bandpass filter) 200. In this way, the intermediate frequency signal Sf containing the analog modulated wave and the digital modulated wave existing in the frequency band of about 400 kHz of a specific channel is selectively retrieved. Further, the intermediate frequency signal Sf retrieved from the IF bandpass filter 200 is amplified to a predetermined level by an intermediate frequency amplifier (hereinafter referred to as the IF amplifier) 300, and input to a demodulator 550.
The intermediate frequency signal Sg output from the IF amplifier 300 is input to an intermediate frequency automatic gain control unit (hereinafter referred to as the IF AGC) 400, and fed back to the IF amplifier 300. The IF AGC 400 has the function of controlling the gain (amplification degree) of the IF amplifier 300 automatically in accordance with the level of the intermediate frequency signal Sg. The gain of the IF amplifier 300 can also be controlled automatically by supplying the IF AGC 400 with the intermediate frequency signal from an analog IF signal processing unit 500 described later.
In the IF bandpass filter 200, the IF amplifier 300 and the IF AGC 400, the intermediate frequency signal containing the analog modulated wave and the digital modulated wave existing in the frequency band of about 400 kHz of a specific channel is retrieved, and the output level of the intermediate frequency signal containing the analog modulated wave and the digital modulated wave is adjusted automatically.
The demodulator 550 of the audio broadcast receiving apparatus shown in FIG. 2, on the other hand, includes an analog-to-digital converter 350 for the intermediate frequency signal (hereinafter referred to as the IF A/D) for converting the analog modulated wave contained in the intermediate frequency signal Sg output from the IF amplifier 300 into the intermediate frequency signal Sdi in digital form, an analog intermediate frequency signal processing unit (hereinafter referred to as the analog IF signal processing unit) 500 for demodulating the digital intermediate frequency signal Sdi and retrieving the analog audio signal Saa, and a digital intermediate frequency signal processing unit (hereinafter referred to as the digital IF signal processing unit) 600 for demodulating the digital modulated wave contained in the intermediate frequency signal Sg output from the IF amplifier 300 and retrieving the digital audio signal Sda through a sampling rate converter 660. This digital IF signal processing unit 600 also decodes the channel selected by the tuner 100 and applies it, as a channel decode signal Sch, to the audio signal mixer 700.
It should be noted that the digital modulated wave contained in the intermediate frequency signal Sg is passed as it is through the IF A/D 350 and the analog IF signal processing unit 500, and input to the digital IF signal processing unit 600 as a quadrature modulation signal of OFDM scheme with the data arranged along I axis (time axis) and Q axis (frequency axis).
Further, the demodulator 550 of the audio broadcast receiving apparatus shown in FIG. 2 includes an audio signal mixer 700 for mixing, by blending, the analog audio signal Saa retrieved from the analog IF signal processing unit 500 with the digital audio signal Sda retrieved from the digital IF signal processing unit 600, and an audio signal processing unit 800 for generating the audio information Sdp containing the intended sound by filtering, with a digital filter or the like, the mixed audio signal Sb mixed by the audio signal mixer 700. The audio signal mixer 700 and the audio signal processing unit 800 are normally configured of a digital signal processor (normally referred to as DSP) for processing the digital signal.
In the digital IF signal processing unit 600, the quadrature modulation signal is demodulated at the sampling rate (sampling frequency) of about 1 MHz. In the analog IF signal processing unit 500, on the other hand, the intermediate frequency signal is demodulated at the sampling rate of about 44 kHz. Before the blending operation by the audio signal mixer 700, therefore, the sampling rate of the demodulation processing signal Spa retrieved from the digital IF signal processing unit 600 must be adjusted to the sampling rate of the analog audio signal Saa retrieved from the analog IF signal processing unit 500. Thus, the demodulation processing signal Spa retrieved from the digital IF signal processing unit 600 is down-sampled by the sampling rate converter 660 and then input to the audio signal mixer 700 as a digital audio signal Sda having substantially the same sampling rate as the analog audio signal Saa.
Further, in the audio broadcast receiving apparatus shown in FIG. 2, the audio information Sdp in digital form retrieved from the audio signal processing unit 800 is converted to the audio information Ad in analog form by the digital-to-analog converter (abbreviated to D/A in FIG. 2) 820. The audio information Ad in analog form retrieved from the A/D converter 820 is finally input to a speaker 840, so that the sound of the analog audio broadcast and the digital audio broadcast of the broadcast station desired by the user is output from the speaker 840.
Furthermore, the audio broadcast receiving apparatus shown in FIG. 2 comprises a control unit 900 for controlling a series of operation of each component element of the audio broadcast receiving apparatus 110 based on various types of control signal Sco. The control unit 900 is normally implemented by a central processing unit (CPU).
Now, with reference to the receiving characteristic diagram of FIG. 3, an explanation is given, about the receiving characteristic, on the assumption that an analog modulated wave (hereinafter sometimes referred to as the analog wave as required) of the analog audio broadcast such as the FM radio broadcast and a digital modulated wave (hereinafter sometimes referred to as the digital wave as required) of the digital audio broadcast are received at the same time using the conventional audio broadcast receiving apparatus shown in FIG. 2.
FIG. 3 illustrates a graph showing the relation between the distance from the broadcast station desired by the user and the quality of the sound received from by the conventional audio broadcast receiving apparatus. As apparent from the graph of FIG. 3, as far as the digital wave containing the digital audio signal is concerned, a digital wave of comparatively high sound quality can be received in the case in which the broadcast station desired by the user is located at a short distance while, with an increase of the distance from the particular broadcast station to more than a predetermined value, the sound quality is sharply deteriorated and the digital wave can no longer be received. With regard to the analog wave containing the analog audio signal, on the other hand, the sound quality of the analog wave received is gradually reduced with the increase in the distance from the broadcast station desired by the user.
In other words, a digital audio signal of comparatively high sound quality can be received in a medium or stronger electric field when the distance is small, from the broadcast station, while the analog audio signal can be received in the weak or lower electric field when the distance is large from the broadcast station.
In the case in which the analog audio signal and the digital audio signal having the receiving characteristic described above are mixed, by blending, in the audio signal mixer 700, the audio signal is digitized by preferentially retrieving the digital audio signal (see the area advantageous for the user in FIG. 3) having a higher sound quality than the analog audio signal in the medium or stronger electric field. As a result, the sound quality in the medium or stronger electric field is improved. In the weak or lower electric field, on the other hand, the digital audio signal cannot be received and is automatically switched to the analog audio signal, and therefore, the service area remains the same as that of the ordinary FM radio broadcast and the AM radio broadcast.
Further, the frequency characteristic of the IF bandpass filter 200 shown in FIG. 2 is explained with reference to FIG. 4. As shown in portion (a) of FIG. 4, only the analog wave existing in the band MB in the frequency band of a specific channel of the FM radio broadcast can be selectively passed by setting the bandwidth of the IF bandpass filter to about 200 kHz. In the conventional audio broadcast receiving apparatus of IBOC scheme, however, both the analog wave existing in the band MB in the frequency band of the particular channel and the digital wave existing in the upper and lower sidebands SB are required to be passed through the IF bandpass filter. As shown in portion (b) of FIG. 4, therefore, the bandwidth of the IF bandpass filter is required to be set to about 400 kHz.
This makes it necessary to make the bandwidth of the IF bandpass filter wider than in the case in which only the analog wave is passed. As described above, the frequency interval between the broadcast station of a given channel and the broadcast station of an adjacent channel is only 200 kHz. As a result, the interference may occur between the broadcast station of the channel desired by the user (hereinafter referred to as the desired station, as required) and the broadcast station of an adjacent channel (hereinafter referred to as an adjacent station, as required).
FIGS. 5 and 6 illustrate first and second schematic diagrams, respectively, for explaining the problems of the prior art. With reference to the schematic diagrams of FIGS. 5 and 6, an explanation is given about the problem points posed in the case in which the analog wave and the digital wave of the desired station are received using the conventional audio broadcast receiving apparatus of IBOC scheme.
As shown in FIG. 5 and portion (a) of FIG. 6, assume that a mobile unit C receiving the analog audio broadcast signal and the digital audio broadcast signal of IBOC scheme transmitted in the frequency band (the central frequency fd of the channel) of the channel of the desired station S-0 is approaching a broadcast station adjacent to the upper sideband (central frequency fd of the channel plus 200 kHz) in the frequency band of the desired station S-0 (situation (1)). Also, assume that the upper adjacent station S-1 transmits only the analog audio broadcast signal.
With the approach of the mobile unit C to the upper adjacent station S-1, the receiving strength of the analog wave of the central band MB-1 in the frequency band of the upper adjacent station S-1 increases. As the mobile unit C moves farther away from the desired station S-0 (partly under the effect of a building H), on the other hand, the receiving field strength of the digital wave of the upper sideband SB-0 in the frequency band of the desired station S-0 decreases. When receiving the digital wave of the upper sideband SB-0 in the frequency band of the desired station S-0, therefore, the analog wave of the band MB-1 of the upper adjacent station S-1 is received at the same time, thereby interfering with the receiving of the digital wave of the desired station S-0. This interference by the upper adjacent station S-1 causes a radio interference between the desired station S-0 and the upper adjacent station S-1 and poses the problem that the digital wave of the digital audio broadcast of the desired station S-0 cannot be received.
Further, the analog wave of the upper adjacent station S-1 intrudes, as noise, into the frequency band portion of the upper sideband SB-0 of the desired station S-0, and therefore, the gain of the IF amplifier 300 (FIG. 2) is adjusted downward automatically by the IF AGC 400. As a result, the receiving strength of the analog wave in the band MB-1 of the desired station S-0 becomes weak, and the analog wave of the analog audio broadcast of the desired station S-0 also becomes difficult to receive. In addition, intermodulation occurs between the analog wave in the band MB-1 of the upper adjacent station S-1 and the analog wave in the band MB-1 of the desired station S-0, thereby posing another problem that the analog wave of the analog audio broadcast of the desired station S-0 cannot be received.
As shown in FIG. 5 and portion (b) of FIG. 6, on the other hand, assume that a mobile unit C receiving the analog audio broadcast signal and the digital audio broadcast signal of IBOC scheme transmitted in the frequency band (the central frequency fd of the channel) of the channel of the desired station S-0 approaches a broadcast station adjacent to the lower sideband (i.e. the lower adjacent station) (central frequency fd of the channel less 200 kHz) in the frequency band of the desired station S-0 (situation (2)). Also, assume that the lower adjacent broadcast station S-2 transmits both the analog audio broadcast signal and the digital audio broadcast signal of IBOC scheme.
With the approach of the mobile unit C to the lower adjacent station S-2, the receiving strength of the digital wave of the upper sideband MB-2 in the frequency band of the lower adjacent station S-2 increases. On the other hand, the receiving field strength of the digital wave of the lower sideband SB-0 in the frequency band of the desired station S-0 decreases. When receiving the digital wave of the lower sideband SB-0 of the desired station S-0, therefore, the digital wave of the upper sideband SB-2 of the lower adjacent station S-2 is received at the same time, thereby interfering with the receipt of the digital wave of the desired station S-0. This interference by the lower adjacent station S-2 causes a radio interference between the desired station S-0 and the lower adjacent station S-2 and poses the problem that the digital wave of the digital audio broadcast of the desired station S-0 cannot be received.
Further, the interference by the lower adjacent station S-2 causes intermodulation between the analog wave in the band MB-2 of the lower adjacent station S-2 and the analog wave in the band MB-0 of the desired station S-0, thereby posing another problem that the analog wave of the analog audio broadcast of the desired station S-0 cannot be received.
As described above, the conventional audio broadcast receiving apparatus of IBOC scheme poses the problem that in the case in which an adjacent station transmits only the analog audio broadcast signal, and in the case in which it transmits the analog audio broadcast signal and the digital audio broadcast signal of IBOC scheme, interference and intermodulation occur between the desired station and the adjacent station, thereby making it impossible to receive the analog audio broadcast and the digital audio broadcast of IBOC scheme from the desired station in stable fashion.
For reference, three patent documents (Patent Documents 1 to 3) containing the description of the techniques relating to the audio broadcast receiving apparatus having the IF bandpass filter are listed below.
Patent Document 1 (Japanese Unexamined Utility Model Publication (Kokai) No. 5-18135) discloses a configuration of the FM receiver in which the lock range of the PLL detection circuit is limited to a smaller value than the IF bandwidth by a band limiter to remove the adjacent interference signal that cannot be removed in the IF band. Patent Document 2 (Japanese Unexamined Patent Publication (Kokai) No. 5-199134), on the other hand, discloses a configuration of the FM receiver in which the noise due to the interference of an adjacent station is removed by switching the IF band amplifier. Also, Patent Document 3 (Japanese Unexamined Patent Publication (Kokai) No. 7-193518) discloses a configuration of the AM receiver in which the resultant output signal obtained by adding a signal output from a wide band IF filter and another signal output from a narrow band IF filter, after changing the ratio of the former signal level with respect to the latter signal level, is used as an intermediate frequency signal.
All of Patent Documents 1 to 3, however, fail to refer to the configuration of the audio broadcast receiving apparatus of IBOC scheme, and have a different object and a different configuration from this invention. Further, in the configuration disclosed in Patent Document 3, unlike the audio broadcast receiving apparatus according to the invention, the wide band IF filter and the narrow band IF filter are not operated independently of each other.